This invention relates to the field of thin film materials used in magnetic disks for data storage devices such as disk drives. More particularly the invention relates to thin film layers used to condition a nonmagnetic substrate for subsequent crystalline layer structures.
The magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based magnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The most common disk in the market is currently made with a substrate disk of AlMg on which a layer of amorphous NiP is electrolessly deposited. The use of the electroless NiP process has several disadvantages including the fact that it is a wet process that must be performed quite separately from the sputtering process by which the remainder of the layers are deposited. It is difficult to achieve the smoothness and uniformity in the NiP surface which is needed for the densities now required for disk drives. The NiP is also a source of corrosion problems and to some degree limits the processing temperatures due to the fact that NiP can become magnetic if heated above a certain point.
The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size and magnetic exchange decoupling between the grains, play key roles in controlling the recording characteristics of the disk. The Cr underlayer is mainly used to influence such microstructural parameters as the PO and grain size of the cobalt-based magnetic alloy. The PO of the various materials forming the layers on the disk is not necessarily an exclusive orientation which may be found in the material, but is merely the dominant orientation. When the Cr underlayer is deposited at elevated temperature on a NiP-coated AlMg substrate a [100] preferred orientation (PO) is usually formed. This PO promotes the epitaxial growth of [11{overscore (2)}0] PO of the hcp cobalt (Co) alloy, thereby improving the in-plane magnetic performance of the disk for longitudinal recording. The [11{overscore (2)}0] PO refers to a film of hexagonal structure whose (11{overscore (2)}0) planes are predominantly parallel to the surface of the film. Since nucleation and growth of Cr or Cr alloy underlayers on glass and most nonmetallic substrates differ significantly from those on NiP-coated AlMg substrates, different materials and layer structures are used on glass substrate disks to achieve optimum results. The conventional NiP coating is not preferable for use on glass for many reasons including those noted above. Nonmetallic substrate disks have typically had a so called xe2x80x9cseed layerxe2x80x9d sputter deposited onto the substrate before the Cr-alloy underlayer. The use of a judiciously chosen seed layer allows the performance of nonmetallic substrates to exceed NiP/AlMg disks. The seed layer influences nucleation and growth of the underlayer which in turn affects the magnetic layer. Several materials have been proposed in published papers for seed layers such as: Al, Cr, CrNi, Ti, Ni3P, MgO, Ta, C, W, Zr, AlN and NiAl on glass and nonmetallic substrates. (See for example, xe2x80x9cSeed Layer induced (002) crystallographic texture in NiAl underlayers,xe2x80x9d Lee, et al., J. Appl. Phys. 79(8), Apr. 15, 1996, p.4902ff). In a single magnetic layer disk, Laughlin, et al., have described use of an NiAl seed layer followed by a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [10{overscore (1)}0] texture in the magnetic layer. (xe2x80x9cThe Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media,xe2x80x9d IEEE Trans. Magnetic. 32(5) September 1996, 3632). In one of the related applications noted above, the use of RuAl for a seed layer is disclosed.
The design of magnetic disks has progressed rapidly in recent years making improvements ever more difficult. In some metrics, e.g. signal-to-noise ratio (SNR), even 1 dB improvement is now considered quite significant. As of the time of this application, the highest claimed recording density on magnetic disks by anyone in the industry is between 30 and 40 gigabits per square inch. This density has been achieved only in the laboratory and the density found in the state of the art commercially available disk drive is far below this value. Thermal stability of the recorded information on the disk is the presumed limiting factor as higher densities are sought. A commercially viable disk drive must be capable of maintaining the stored information for periods of time measured in years.
Chen, et al., have recently described experimental results on disks with what they call a Cr sub-seed layer. Glass substrates with a sputtered NiP layer were used to receive the Cr/NiAl/Cr-alloy/Co-alloy layer structure. The article is silent as to the desired crystal structure of the Cr sub-seed layer. (See Q. Chen et al, IEEE Transactions on Magnetics, vol. 35, no. 5, page 2637, September 1999).
The thin film disk of the invention includes a thin film pre-seed layer of amorphous or nanocrystalline structure. The pre-seed layer which may be chrome-tantalum (CrTa) or aluminum-titanium (AlTi) is deposited prior to first crystalline layer. Although the pre-seed layer may be amorphous or nanocrystalline, for brevity it will be referred to herein as amorphous which is intended to encompass a nanocrystalline structure. In the preferred embodiment the pre-seed layer is sputtered onto a nonmetallic substrate such as glass, followed by a ruthenium-aluminum (RuAl) layer with B2 structure. The use of the pre-seed layer of the invention improves grain size and its distribution, in-plane crystallographic orientation and coercivity (Hc) and SNR. In a preferred embodiment the pre-seed is followed by the RuAl seed layer, a Cr alloy underlayer, an onset layer and a magnetic layer. The amorphous pre-seed layer also allows use of a thinner RuAl seed layer which results in smaller overall grain size, as well as, a reduction in manufacturing cost due to relatively high cost of ruthenium. The increased coercivity also allows use of a thinner Cr alloy underlayer which also contributes to decreased grain size. Another benefit lies in the fact that the pre-seed layer provides additional thermal conductivity which could help prevent thermal erasures on a glass disk.